This invention relates generally to a method and apparatus of controlling an engine, and more particularly, to an apparatus and method configured to maintain a desired emissions level of an engine.
Engine emissions, such as NOx emissions, play an important role in engine control. In some applications, emitting higher than desired NOx levels, while still within regulatory standards, may cause problems in the particular application the engine is being used. For example, in a greenhouse a low level of NOx is desirably maintained at an even level. However, current control systems are unable to do this. If the engine emissions are higher than a designated amount, then the emissions adversely effect the greenhouse. On the other hand, if the engine emissions are below the designated amount, then overall engine performance may suffer. That is, engine efficiency decreases as NOx emissions levels decrease. Therefore running the engine in an operating range where lower NOx levels are being emitted than necessary to meet site or regulatory emissions restrictions, causes a reduction in engine operating efficiency.
Changes in the ambient conditions may have a significant impact on the NOx emissions, and in particular the ability to maintain the NOx emissions at a desired level. For example, as the specific humidity increases in the air within the intake manifold, the higher water content in the intake air reduces the peak combustion temperature, and therefore reduces the NOx formation. In addition, the higher specific humidity means there is less oxygen in the cylinder during combustion, and therefore less oxygen exhausted from the cylinder. Both of these issues lead to a reduced oxygen content in the exhaust stream of the engine. Without accounting for the changes in the ambient conditions, the reduced oxygen content may be misinterpreted by a control algorithm which may either unnecessarily adjust the air fuel ratio, or adjust the air fuel ratio in the wrong manner, causing decreased performance in the engine.
Some systems calculate a specific humidity, and use the specific humidity to modify the determined lean limit of the engine. However, operating the engine at a lean limit, and modifying the lean limit to account for changes in the specific humidity does not address the problem of operating an engine in a manner to maintain a desired emissions level despite changes in the ambient conditions, such as specific humidity and/or exhaust pressure.
The present invention is directed to overcoming one or more of the problems set forth above.
In one aspect of the present invention, a method of maintaining a desired emissions level of an engine having an intake manifold and an exhaust manifold, and an exhaust stack is disclosed. The method includes the steps of establishing a desired emissions level, establishing an engine speed, establishing an engine load, establishing at least one characteristic of one of an intake air and an exhaust gas, and determining a fuel command in response to the engine speed, the engine load, and the desired emissions level, the fuel command resulting in the engine maintaining the desired emissions level.